1. Field of the Invention
The present invention relates to transmitting and receiving data and, more particularly, to a method for improving data transmission efficiency when data corresponding to the same logical channel is broadcast from each base station that has respectively different channel environments.
2. Background of the Related Art
In a mobile communications system that supports broadcast/multicast services, multimedia data including images need to be transmitted in addition to voice data, thus a high data rate is required. Accordingly, to provide broadcast/multicast services, a packet data channel of the physical layer should be able to support high data rates.
In a wireless environment where fading exists, in order to transmit multimedia data through the packet data channel in a stable manner, the Hybrid Automatic Repeat Request (HARQ) scheme is applied. HARQ combines the techniques of Forward Error Correction (FEC) and Automatic Repeat Request (ARQ).
The HARQ scheme will be explained in more detail as follows. First, for the data to be transmitted, encoding is performed by using a channel coder (e.g., a turbo encoder) having an error correction function, and one or more sub-packets associated with a single packet are transmitted.
When a first sub-packet is transmitted from the transmitting end, decoding is performed at the receiving end that received the first sub-packet. If decoding is successfully performed, an acknowledgement (ACK) signal is sent to the transmitting end. Meanwhile, if decoding of the received first sub-packet is not successful, a negative acknowledgement (NACK) signal is fed back to the transmitting end.
At the transmitting end, if an ACK signal is received, a first sub-packet associated with a subsequent packet is transmitted. If a NACK signal is received, a second sub-packet associated with the packet that was already transmitted is transmitted. At the receiving end, the first sub-packet is stored in a buffer, and when the second sub-packet is transmitted, the first and second sub-packets are combined and decoding is performed such that the success rate of decoding can be increased.
FIG. 1 shows an exemplary method of implementing HARQ to a packet interlace structure. Referring to FIG. 1, the channel used for transmitting packet data can be implemented by using a structure whereby each interlace is regularly repeated with a certain time interval. As shown in FIG. 1, the exemplary packet data channel include four interlaces, thus a single packet is transmitted by employing one of the four interlaces. When the interlace to be transmitted is determined, the corresponding packet is transmitted through the determined interlace. This will be explained in more detail as follows.
In FIG. 1, for the 0th packet, the first sub-packet is P00, the second sub-packet is P01, the third sub-packet is P02, and the fourth sub-packet is P03. For the 1st packet, the first sub-packet is P10.
As shown in FIG. 1, it is assumed that the 0th packet is transmitted by using the 0th interlace. From the transmitting end, a first sub-packet associated with the 0th packet is transmitted to the receiving end via the 0th interlace. Upon receiving and decoding the first sub-packet at the receiving end, if decoding was unsuccessful, a NACK signal is fed back to the transmitting end. At the transmitting end, upon receiving the NACK signal, the 0th interlace is used to transmit a second sub-packet associated with the 0th packet to the receiving end. Upon receiving the second sub-packet, the receiving end combines the second sub-packet with the first sub-packet that was stored in a buffer and decoding is performed. Despite this, if decoding is still unsuccessful, a NACK signal is fed back to the transmitting end.
At the transmitting end, upon receiving the NACK signal, the 0th interlace is used again to transmit a third sub-packet associated with the 0th packet to the receiving end. This procedure is repeatedly performed until an ACK signal is received or until a threshold number of times is reached. As above, each sub-packet associated with a single packet is transmitted by using the same interlace.
When transmitting broadcast/multicast data through a packet data channel, the above ACK/NACK feedback does not exist. This is due to the characteristics of one-to-many communications that is characteristic of broadcast/multicast services. Accordingly, when broadcast/multicast data is transmitted, because individual ACK/NACK signals cannot be received with respect to each mobile station, a transport format must be determined such that any mobile station existing in a particular cell has a reception quality that is above a certain threshold. Such transport format comprises a data rate, payload size, the number of transmitted sub-packets, the modulation method to be used, etc. When this transport format has been determined, each base station performs broadcast/multicast services according to the determined transport format.
FIG. 2 shows exemplary types of data rates that can be provided when the number of sub-packets are changed while the payload size and modulation method are fixed. Here, it is assumed that the payload size is 2048 bits, and the sub-packet transmission time interval is 1/600 seconds. As shown in FIG. 2, upon considering the fading environment, the interference environment, cell radius, etc., for a base station having a good overall channel environment (conditions), the 0th interlace is used once to transmit a single packet, thus broadcast/multicast packets can be transmitted at a high data rate (e.g., 1.2288 Mbps). However, if the overall channel environment (conditions) is not good, the 0th interlace is used 4 times to transmit a single packet, thus broadcast/multicast packets are transmitted at a low data rate (e.g., 307.2 kbps).
The broadcast/multicast data is transmitted via a packet data channel having an interlace structure, and each interlace has at least one multiplex. Preferably, a single interlace includes 4, 8, or 16 multiplexes. Thus, an interlace-multiplex pair is used to indicate which multiplex within which interlace is used to transmit a packet.
For each interlace-multiplex pair, there is a burst length associated thereto. The burst length is determined by multiplying the number of sub-packets per packet according to a transmission data rate by the number of packets to be transmitted per burst. An interlace-multiplex pair consecutively occupies a particular interval of the same interlace that equals a burst length. Accordingly, the packet data channel, through which broadcast/multicast data is transmitted, comprises sub-channels defined by interlace-multiplex pairs. The base station maps one logical channel that includes at least one broadcast/multicast service (BCMCS) flow to at least one interlace-multiplex pair.
Table 1 shows an example of an overhead signaling message that includes information related to an interlace-multiplex pair, burst length, and the number of sub-packets per packet.
TABLE 1FieldLength (bits)[ . . . ]Interlace0Included1SameBurstLengths00 or 1TotalBurstLength0 0 or 10Zero, one, or MultiplexesPerInterlace-1 occurrences of the followingfield:BurstLength04Interlace1Included1SameBurstLengths10 or 1TotalBurstLength1 0 or 10Zero, one, or MultiplexesPerInterlace-1 occurrences of the followingfield:BurstLength14Interlace2Included1SameBurstLengths20 or 1TotalBurstLength2 0 or 10Zero, one, or MultiplexesPerInterlace-1 occurrences of the followingfield:BurstLength24Interlace3Included1SameBurstLengths30 or 1TotalBurstLength3 0 or 10Zero, one, or MultiplexesPerInterlace-1 occurrences of the followingfield:BurstLength34[ . . . ]Zero or one occurrence of the following four fields:PhysicalChannelCount7DataRate0 or 4OuterCode0 or 4MACPacketsPerECBRow0 or 4Zero or PhysicalChannelCount occurrence of the following two fields;Interlace2Multiplex4
In Table 1, the Interlace0Included, Interlace1Included, Interlace2Included, and Interlace3Included fields indicate which interlace is used for the BCMC service. For example, if the 0th interlace is used for the BCMCS, the Interlace0Included field corresponding to the 0th interlace is set to ‘1’, but can be set to ‘0’ if not used.
The MultiplexesPerInterlace field indicates the number of multiplexes that comprise one interlace.
Also, the BurstLength0, BurstLength1, BurstLength2, and BurstLength3 fields indicate the burst length corresponding to each interlace-multiplex pair, respectively.
Additionally, the PhysicalChannelCount field indicates the number of physical sub-channels, whereby a sub-channel refers to an interlace-multiplex pair used to transmit one BCMCS logical channel.
The DataRate field indicates the data rate of the corresponding physical channel. Here, according to this data rate value, the size of a packet transmitted through the corresponding physical channel and the number of slots needed to transmit one packet are determined.
Table 2 shows an example of data rates according to the DataRate field value of Table 1 and the number of slots needed to transmit the packets transmitted through the corresponding physical channel.
TABLE 2Slots per BroadcastDataRatexxx fieldData RatePhysical Layer packet‘0000’ 38.4 kbps16‘0001’ 76.8 kbps8‘0010’153.6 kbps4‘0011’204.8 kbps3‘0100’307.2 kbps2‘0101’308.2 kbps4‘0110’409.6 kbps3‘0111’614.4 kbps1‘1000’614.4 kbps2‘1001’921.6 kbps2‘1010’12288.8 kbps 1‘1011’12288.8 kbps 2‘1100’1843.2 kbps 1‘1101’2457.6 kbps 1‘1110’ to ‘1111’Reserved
In Table 1, the interlace field and the multiplex field are used for the purpose of informing about which interlace-multiplex pair the corresponding physical channel is transmitted through. Here, the number of packets associated with one interlace-multiplex pair can be calculated by dividing the burst length by the number of slots associated with one packet.
FIG. 3 shows an exemplary method of transmitting broadcast/multicast data using an interlace-multiplex pair structure. When the interlace-multiplex pair is indicated as (interlace number, multiplex number), broadcast/multicast data is transmitted through the packet data channel as shown in FIG. 3. Here, FIG. 3 shows an example where there are four multiplexes per each interlace, and each multiplex burst length is one.
FIG. 4 shows exemplary zone structures for a broadcast/multicast service. A broadcast/multicast service may be provided through a zone-based manner (Zone A through Zone G). In a zone-based service, a region occupied by at least one base station group is defined as one zone unit, and for each zone unit, a BCMCS flow is provided as a service, independently. Accordingly, the base stations, which are part of the same zone, transmit upon mapping the same logical channel having the same BCMCS flows to the same interlace-multiplex pair.
It should be noted that each zone may be comprised of smaller regions or areas called cells or sectors. Here, a cell may be a region defined on the basis of terrain characteristics, while a sector may be a region defined on the basis of signal characteristics. Also, a base station may manage one or more cells, or one or more sectors. As such, it can be said that a service is provided on a per cell basis, on a per sector basis, on a per base station basis, or the like. The following description will generally refer to zones having cells therein merely for the sake of simplicity.
As above, when performing a zone-based broadcast/multicast service, all base stations within each zone transmit the same data through the same interlace-multiplex pair. Thus, a mobile station located within a particular zone, receives the same packet transmitted from at least one base station within that same zone, and by combining and decoding these, diversity gain may be obtained.
FIG. 5 shows an example for explaining cells having respectively different channel environments within a single zone. Like cell A, for a mobile station in a cell located at a central region of a zone, the same packet being transmitted from neighboring base stations is received, and diversity gain can be obtained. However, like cell B or cell D, for a mobile station in a cell located at an outer periphery of the zone, other packets transmitted from cells that are part of other zones cause interference and the channel state (condition) may thus be no good. Meanwhile, like cell C, although located at a central region of the zone, the channel state (condition) may be poor due to environmental characteristics of the cell itself, due to terrain, buildings, or the like.
Accordingly, in a cell with good channel conditions (such as cell A), packets can be transmitted at a high data rate, such as 1.2288 Mbps. But in locations where the channel conditions are not good (such as cells B, C, or D), to overcome poor channel conditions, redundancy information is added and because such needs to be transmitted multiple times, packets can only be transmitted at 614.4 kbps or at a lower data rate.
FIG. 6 shows an exemplary method of transmitting broadcast/multicast data according to the related art. Referring to FIG. 6, one interlace includes four multiplexes, and the BCMCS logical channel is mapped to four interlace-multiplex pairs (0,0), (0,1), (0,2), (0,3). Namely, a method of transmitting data using the entire 0th interlace is shown. Here, the burst length is 1. In the first embodiment, because the burst length is 1, only one sub-packet per one packet is transmitted. Thus, although packets can be transmitted at a high data rate, degraded service quality with respect to a mobile station located in a cell with poor channel conditions is one type of problem that occurs.
FIG. 7 shows another exemplary method of transmitting broadcast/multicast data according to the related art. Referring to FIG. 7, a single interlace includes four multiplexes, and a BCMCS logical channel is mapped to four interlace-multiplex pairs (0,0), (0,1), (0,2), (0,3). Namely, a method of transmitting data using the entire 0th interlace is shown. Here, the burst length is 3. In this case, because the burst length is 3, three sub-packets per one packet may be transmitted. Thus, in a zone-based structure, with respect to mobile stations located in a cell with poor channel conditions, service quality may be guaranteed in a more stable manner, but a low data rate is another type of problem that occurs.
As explained above, when providing zone-based broadcast/multicast services, the channel conditions for each cell may be different. However, if the transport format is determined on the basis of a particular cell, a waste of radio (wireless) resources occurs with respect to each cell, which can cause a decrease in efficiency and a decrease in service quality.